Photo-lithography is widely used in the semiconductor industry to form a wide range of structures in integrated circuit chips. As the size of the chips decrease and their density increases, optical phenomena, such as diffraction, interference and light divergence, becomes increasingly important for they can adversely affect the resolution of the photolithography rendering further reduction in size and increases in density more difficult to realize.
To minimize the importance of such phenomena and extend the range of photolithography a technique known as phase-shift, based on phase destructive interference of the waves of incident light, was developed.
Phase-shift lithography shifts the phase of one region of incident light waves approximately 180.degree. (.pi. radians) relative to an adjacent region of incident light waves to create a more sharply defined interface between the adjacent regions than is otherwise possible.
when phase-shifting is used with semiconductor photomasks the boundary between exposed and unexposed portions of a photoresist exposed through the mask is more closely defined. By more closely defining the boundaries, the circuit created thereby can be smaller and closer. Thus, a greater circuit density can be realized.
A number of phase-shift lithography techniques have been developed. One of the earliest is reported by Levenson et al. in the article entitled "Improved Resolution in Photolithography with a Phase-shifting Mask" IEEE TRANSACTIONS ON ELECTRON DEVICES, Vol. ED-29 No. 12, December 1982, Pages 18-36. The technique disclosed in this article uses a periodic pattern arrangement in the transmission mask and provides a sharp image contrast. Practical semiconductor integrated circuit masks using the described technique are however difficult to fabricate for the mask openings must be arranged in a periodic fashion. Such periodic arrangements are generally not suitable for the more complex, present day, integrated semiconductor circuits.
In an attempt to avoid the relatively complex mask fabrication techniques required by the Levenson approach, an alternative process known as the self aligned or rim type phase-shift process was developed and has been reported by Todokoro et al. in an article entitled "Self-aligned Phase-shifting Mask for Contact Hole Fabrication" appearing in MICROELECTRONIC ENGINEERING, No. 13, 1991, Pages 131-134 and by Ishiwata et al. in the article entitled "Fabrication of Phase-shifting Mask." which appeared in the PROCEEDINGS OF THE SPIE, Vol. 1463, 1991, Pages 423-433.
Although such rim phase-shifting masks are thus inherently applicable to arbitrary masks patterns and overcome the limitations of the Levenson approach, they are also limited for they require a large positive mask bias to reduce exposure times to reasonable levels and also have strong proximity effects. This makes it difficult to utilize single mask patterns designed to expose in a single common exposure a variety of feature sizes and shapes.
To overcome the above described limitations of rim phase-shifting masks, the attenuated phase-shifting mask was developed, as reported by Burn J. Lin in an Article entitled "The Attenuated Phase-Shifting Mask" that appeared in the January 1992 Issue of SOLID STATE TECHNOLOGY, Pages 43-47.
Phase-shift lithography, including attenuating phase-shift lithography, is thus based on and takes advantage of the well known opposite phase, destructive interference phenomena to more clearly delineate fine images.
The attenuated phase-shifting masks described by Lin replace the opaque parts of the prior art masks with a slightly phase-shifting transmissive absorber, i.e., an attenuating material, which will provide an approximately 180.degree. (.pi. radians) phase-shift characteristic as contrasted to the adjacent fully transmissive adjacent areas. This absorber is deposited on the surface of the mask adjacent the fully transmissive areas of the mask. These attenuated phase-shift masks require only a small positive mask bias resulting in improved resolution and shorter exposures and, compared with rim phase-shifting masks, have a greater depth of focus, and use less mask areas. However, these attenuated phase-shifting masks, although appreciably better than the rim type phase-shifting masks, are susceptible to process created phase defects in the printing regions of the mask .
Furthermore, not all absorbers can both phase-shift and absorb by the desired amount without first modifying the surface of the substrate to provide, in the absorbing or attenuating regions, an additional transparent phase-shifting layer that will compensate for the insufficient phase-shifting of the absorber. This modification of the surface however requires a treatment, such as etching of the surface of the printing or fully transmissive areas of the substrate, which introduces the undesirable, process created, phase defects discussed above.
Accordingly, there now exists a need for an improved attenuating mask which is not susceptible to undesirable phase defects in the printing areas of the mask while still providing all the advantages that the attenuation masks have over all other types of phase-shifting masks.